Pressure gradient web cleaning method



April 1969 T. A. GARDNER 3,436,265

PRESSURE GRADIENT WEB CLEANING METHOD Original Filed Aug. 19, 1963 Sheetof 2 April 1, 1969 'r. A. GARDNER 3,436,265

PRESSURE GRADIENT WEB CLEANING METHOD Original Filed Aug. 19, 1963 Sheet4 of 2 My '7 U INVENTOR.

fidnrlxlazoamc United States Patent 3,436,265 PRESSURE GRADIENT WEBCLEANING METHOD Thomas A. Gardner, 513 Clark St, Neenah, Wis. 54956Original application Aug. 19, 1963, Ser. No. 302,978, now Patent No.3,239,863. Divided and this application Jan. 8, 1965, Ser. No. 424,246

Int. Cl. 1308b 3/02 U.S. Cl. 13437 3 Claims ABSTRACT OF THE DISCLOSUREThe cleaning of a longitudinally moving web is accomplished by directinga transverse gas jet of at least 10,000 f.p.m. normally against the weband a second transverse gas jet at an acute angle to the movement ofsaid web and toward said first-mentioned jet in the direction of webmovement, and collecting from the vicinity of the web between the gasjets the gas from the secondmentioned jet and that component of the gasof the firstmentioned jet which travels counter to the direction of webmovement. The first-mentioned gas jet originates at a distance from theweb which is no more than 1.0 inch therefrom and the ratio of its crosssectional area at its origin to the distance of its point of origin fromthe web is within the range of 0.02 to 1 and 0.15 to 1. The angle of thesecond gas jet with respect to the web is within the range of 5 todegrees.

This invention relates to pressure gradient web cleaning method. Thisapplication is a division of my copending application Ser. No. 302,978,filed Aug. 19, 1963, now Patent No. 3,239,863.

Dust, lint and other free particles on the surfaces of webs of paper orfoil or plastic have always presented a problem in numerous industriesbecause of the practical diificulties in the way of removal of suchparticles by any apparatus or methods heretofore known.

For example, dust, lint, and other particles of dirt are generated inpaper making processes. Lint and hair commonly originate from press anddryer felts. Cellulose flour and fiber originate from paper in thedrying, calendering, and trimming or cuting operations in paper making.Much of the dust so formed adheres to the surface of the paper becauseof oily or moist surfaces on the particles, or electrostatic charge, orbecause the particles are wholly or partially immersed in the viscousboundary layer film of air on the surface. Usually a relatively largeforce is required to remove the particles.

Coating equipment presents another example. Most types of coatingmachines for applying clay coatings to paper depend on the action ofrolls, rods, or steel blades to smooth out the wet coating on the papersurfaces. Dust and dirt are picked up by coating rolls or caught underrods and blades causing marking and streaking of otherwise smoothcoating. Dust is often picked up by the coating applicator andcirculated in the liquid coating system causing clogging of filters,loss of coating, and higher maintenance costs.

In the printing industry, dust is a problem in all types of printing. Inthe case of letterpress printing, dust clogs up type faces and hard dirtparticles have even broken the printing plates. Offset press blankets,being moistened with water and ink, are particularly susceptible to dustthat tends to build up on the blankets until the press is forced to shutdown for washup. Intaglio printing is also affected by dust that blursprinting, clogs doctor blades, and transfers into the ink wells.

Neither vacuum devices nor brushes nor jets as here- 3,436,265 PatentedApr. 1, 1969 tofore known can effect dislodgement of the objectionableparticles without using force sufliciently great to cause damage to thework or distribution of the objectionable particles within the ambientair around the apparatus. Employing suflicient vacuum to pick up thedust by any previously known procedures would displace the web uponwhich the work is being done and would in itself be a source of damageto the work.

The object of this invention is to provide an effective web cleaner andmethod, the preferred equipment being compact in size, simple tooperate, economical to build and operate, safe to use on any webmaterial, and capable of removing dust of finer particle size than hasheretofore been possible.

The present invention accomplishes the objective of dust removal withoutdamage to the work by disrupting the boundary layer of air on thesurface of the work.

A variety of devices may be used to practice the method of the presentinvention. The preferred equipment is very simple, consisting of aplenum chamber from which two air slot nozzles open toward the work, thespace between these nozzles being enclosed to constitute an exhaustchamber for the air supplied through the nozzles and dust displaced fromthe work.

The primary nozzle is arranged to jet the cleaning air upon the face ofthe work substantially at right angles to the work and sufficientlyclose thereto to produce an extremely narrow impingement Zone. Not onlyis the angle important but the orifice width in relation to its distanceto the web is important as will hereinafter be shown.

About one-half of the air supplied to this nozzzle escapes along thesurface of the web in the direction of web travel and is not picked upin the exhaust chamber. However, this air is clean and therefore doesnot distribute dust into the surrounding atmosphere. The other half ofthe air from the primary nozzle establishes a pressure gradient whichresults in disruption of the boundary layer on the surface of the workand therefore frees dust and other foreign matter and carries it 011..

Before the work reaches the primary nozzle, it passes beneath a nozzlewhich creates a jet of air directed angularly toward the surface of theweb and toward the pri mary nozzle. This nozzle is herein designated asa secondary nozzle because, even though it is the first nozzle beneathwhich the work passes, its function is secondary to that of the nozzlewhich disrupts the boundary layer. The jet from the secondary nozzle maydislodge a relatively small percentage of relatively free particles butits primary function is to seal the chamber against loss of dust-ladenair from the primary nozzle and to contribute to the establishment of asteep pressure gradient at or immediately adjacent the primary nozzle.Substantially all of the air from the secondary nozzle is intercepted bythe exhaust chamber. The distance between nozzles is not critical but islimited by practical considerations.

Air velocities of 10,000 f.p.m. to 40,000 f.p.m. are contemplated but arelatively low volume of air is used and the arrangement is such that nodamage is done to the work even if the web is a very light film.

In the drawings:

FIG. 1 is a view of apparatus for the practice of the invention as itappears in front elevation, portions being broken away.

FIG. 2 is a diagram of a circulatory air system in which such a deviceas that shown in FIG. 1 is preferably incorporated.

FIG. 3 is a view taken in cross section on the line 33 of FIG. 1.

FIG. 4 is a fragmentary detail View showing an alternate secondarynozzle construction.

FIG. 5 is a diagram showing the air flow along the surface of the work,the nozzles and the work being fragmentarily illustrated.

FIG. 6 is a diagram greatly enlarged as compared with FIG. 5 and showingrelative tensions of the boundary layer on the surface of the work asaflected by the primary jet.

FIG. 7 is a fragmentary diagrammatic view taken in cross section likeFIG. 3 and showing a modified embodiment of the invention.

FIG. 8 is a fragmentary diagrammatic view like FIG. 3 showing a furthermodified embodiment of the invention.

FIG. 9 is a fragmentary diagrammatic view like FIG. 3 showing anothermodified embodiment of the invention.

FIG. 10 is a fragmentary diagrammatic view similar to FIG. 3 and showinganother modified embodiment of the invention.

As best shown in FIGS. 1 and 3, preferred equipment for the practice ofthe invention comprises a housing 6 which contains a plenum chamber 8supplied with air through duct 10 for discharge through the primarynozzle 12 and the secondary nozzle 14.

The orifice 16 of the primary nozzle 12 is conveniently made by holdingan outer wall 20 and a partition wall 22 in proper convergingrelationship defined b spacers 18. In practice, these spacers are usedevery two and one-half inches and are offset upwardly from the orifice16 as clearly appears in FIG. 3. The nozzle opening 16 preferablyextends the full width of the work without any substantial interruption.The walls 20 and 22 desirably converge at approximately 10 degrees ormore toward the orifice and the spacers 18 are remote from the orifice16 by an amount at least equal to one spacer diameter. After passing thespacers, the air stream flowing to the nozzle outlet tends to becomerectified so that the jet is virtually uninterrupted across the entirewidth of the work. The nozzle 12 is designed to project a primary jet ofair against the surface of the work in a transverse plane substantiallynormal thereto.

The nozzle orifice width in relation to its distance from the work isimportant. An excessively small ratio is wasteful of energy and loseseffectiveness whereas an excessively large ratio is also wasteful ofenergy and requires quantities of air so great as to be impractical. Thepreferred ratio is 0.10 to 1. Ratios in a range between one extreme of0.02 to 1, and another extreme of 0.15 to 1 are operative. The objectiveis to produce maximum impingement velocity with the least expenditure ofenergy and without excessive flow of air. Increase of distance from theweb requires a proportionate increase in nozzle orifice with resultingincrease in air requirements if the best operation is to be maintained.A small impingement area is essential to the desired effect for reasonshereinafter explained.

The nozzle cannot be so close to the work that it might be contacted bythe moving web, with resultant damage to the work. Neither can theorifice be so small that it will be difficult and costly to make. Inpractice a spacing of the nozzle from the work at about 0.3 inch isoptimum and spacings in a range between 0.2 inch and 1.5 inches arewithin practical limits.

The secondary nozzle 14 directs a secondary jet at an acute angle to thework as will clearly appear from FIG. 3. The outer wall 26 mayconstitute a flange on the side wall of housing 6. The inner wall 28 mayconstitute a flange on the partition 30. The spacers 32 are similar tothose shown at 18 in the primary nozzle and they are similarly remotefrom the nozzle orifice 34. By way of example, and not by way oflimitation, this nozzle is shown to be set at an angle of about 20degrees to the surface of the work and the length of the inner wall 28of the nozzle is about one and one-fourth inches in practice.

The purpose of the angled nozzle is primarily to block the momentum offlow from nozzle 16 along the surface of the work in what may becharacterized in an upstream direction (to the right as viewed in FIG.3). But for this, air from the primary jet would escape from the exhaustchamber upstream along the web. Also, the cleaning effect of the primaryjet would be greatly reduced.

Both the size of Orifice 34 and the angle of impingement are critical.If the angle exceeds degrees, substantial amounts of air will blow outinto the ambient atmosphere, carrying much dust with it. If the angle istoo small, it will similarly permit flow beneath the secondary jet. Thiswill discharge dust into the ambient atmosphere. In normal practice theangles of nozzle 14 to the work would not exceed about 25 degrees andthe minimum practicable angle is about 5 degrees.

The orifice size is important because the momentum of the jet shouldsubstantially balance the momentum of flow along the surface from theprimary nozzle so that neither is capable of overcoming the other tocause air to blow out of the exhaust chamber 36 as defined by partitions22, and 38. The range of appropriate sizes for the orifice 34 is fromfifty to one hundred percent of the size of the orifice 16 whichproduces the main jet. A ratio of sixty-seven percent is consideredoptimum.

An alternate arrangement giving somewhat comparable results is shown inFIG. 4 in which the flange 260 is like flange 26 and the partition 300simply terminates in properly spaced relation to the flange 260 leavingan orifice at 340. Tack welds (not shown) on two and onehalf inchcenters will hold the parts in proper relationship. If the flange 260 isat 22 degrees to the web, the resulting jet will be directed atapproximately the preferred angle of 20 degrees, as in the structureshown in FIG. 3.

The exhaust chamber 36 has one or more ducts 40 opening from it. It isnot necessary that the air be recirculated but this may be a verydesirable procedure, particularly if temperature or moisture or otherfavorable components should advantageously be preserved. In thepreferred system shown in FIG. 2, the discharge ducts 40 are connectedby lines 42 with a line 44 which opens into the filter plenum 46. Makeupair is likewise admitted to the plenum 46 through an opening 48controlled by damper 50.

All such air passes through a deep mat air filter diagrammatically shownat 52 en route to a high pressure blower or other fan 54 having anoutlet line 56 leading to the supply duct 10.

A relatively high degree of vacuum in the exhaust chamber 36 in lieu ofthe secondary jet would overcome any tendency for air and dust to bedischarged into the ambient atmosphere but it is impractical to use anysubstantial degree of vacuum because the resulting pressure differentialon the web will cause much difficulty in the handling of the web. In theinstant device only very slight vacuum is required to prevent loss ofair at the ends of the housing 6. The damper 50 is preferably adjustedto produce only such vacuum as will cause a slight inflow at the ends,an ideal arrangement being one in which the pressures are substantiallybalanced so that there is neither inflow nor outflow.

The spacing between the primary nozzle 12 and the secondary nozzle 14 isnot at all critical. The nozzles should not be so far apart that theenergy of flow is materially dissipated before the flow of therespective jets meet each other.

The work is here representd by a web 60 which may be assumed to bemoving from right to left as viewed in FIG. 3. While the blower 54 ischaracterized as a high pressure fan or blower, the velocity of airmovement to the respective nozzles is relatively low. High velocity atthe jets is achieved by restrictions at the nozzles, 10,000 f.p.m. to40,000 f.p.m. being contemplated. Only about three and one-half poundsof air pressure in the plenum 8 is required to produce a jet velocity of40,000 f.p.m.

Since the pressure in the exhaust chamber 36 is very close to ambientpressure, the flow of the main jet from nozzle 12 will divide nearlyequally on the surface of the work 60, half flowing to the left (FIG. 3)into the ambient atmosphere and half to the right into the exhaustplenum 36. Since the web is clean after it passes under the center lineof the primary jet, the air which blows into the room is clean air andsubstantially free of dust. The part of the primary jet flow whichenters the exhaust chamber 34 will carry substantially all dust from theweb. Substantially all flow from the angled secondary jet produced bynozzle 14 Will enter the exhaust chamber 36, along with any of therelatively loose foreign matter picked up by this jet.

As the moving web 60 carries particles of dust into the cleaner, theparticles are first subjected to the high velocity jet produced bynozzle 14. The more adherent particles will not be dislodged by such ajet but as the work progresses toward the primary nozzle 12 theparticles will encounter progressively increasing velocity andprogressively smaller boundary layer thickness until, at the edge of theimpingement zone directly beneath the main jet, the boundary layer isalmost eliminated and the full impingement velocity of the main jet isflowing parallel to the work at an infinitesimally small distance fromthe Work. This is shown diagrammatically in FIGS. 5 and 6.

The primary jet 120' issuing from primary nozzle 12 is directed normallywith high velocity onto the surface of the work. At the work the jet isdivided into two approximately equal streams 68 and 70, flowing at highvelocity in opposite directions away from the impingement zone 62 alongthe surface of the work. The flow continuously expands from the primarynozzle to the work and from the impingement zone to the point of exhaust72, and progressively loses velocity as the cross section of flowincreases.

All real fluids have viscosity which causes fluid particles to cling tosurfaces and to each other. Thus when a fluid flows along a surfaceboundary, the particles in contact with the surface cling to it, and theparticles once removed from the surface cling to the particles incontact, and so on. As a result a region of reduced velocity is createdadjacent to the surface. This region is called the boundary layer film.Velocities within the film relative to the surface vary from zero at thesurface to full stream velocity at the full depth of the film as shownin the local velocity contour on the righthand side of FIG. 6.

The trick in increasing heat transfer or in picking up dust is to find away to make this sticky viscous film thin so that full stream velocitiesare brought as near to the surface as possible. Then any particle thathappens to be bigger than the thin boundary layer is exposed to highvelocity drag trying to lift it along.

In the impingement zone, the surface area equal to the cross section offlow approaching the surface from an impinging jet, there is a pressurecreated on the surface that is a maximum at the stagnation point 62, andfalls to atmospheric at the edge of the zone. I have found that the rateof change of that pressure affects the boundary layer thickness in thezone. Thus it is desirable to obtain as high a pressure gradient aspossible. The width of the impingement zone is not the width 74 of thenozzle as shown in FIG. 6, but the width of the impinging flow, which isgreater because of jet expansion. As is true of all flow from a nozzle,the jet expands out at an angle of about 7 degrees. Consequently,depending on the spacing of the jet from the work, the resultingimpingement zone and the thinned boundary layer areas 64, 66 will bewider than the nozzle.

The thinness of the boundary layer 64, 66 in the impingement zone isimportant to the success of this web cleaner. Within the impingementzone and starting from the stagnation point of the jet, both thepressure gradient and velocity increase at a rate approximately linearlyrelated to distance from the stagnation point. An approximation of theboundary layer thickness drawn from the Navier- Stokes equations(chapter 9, page 14, Handbook of Fluid Dynamics, 1st ed. (1961)) usingthe above conditions shows that the thickness is constant in that regionand its size depends on the pressure gradient, and the impingementvelocity. It is thus clear that the size of the impingement area must bekept as small as possible for any given impingement velocity in order toincrease the pressure gradient in the impingement area. It can thus beshown that the boundary layer under an optimum nozzle orifice A" awayfrom the surface develops a thickness 50% thinner than an optimum nozzleorifice l" away and requires only 25% as much air energy in doing so.Beyond a practical maximum distance of 1" the jet is so far diffused asto be relatively ineffective. However, it may be as much as 1 /2" awayif air velocity is adequately high, but this is not usually practicable.

The diagram of the boundary layer under the main jet shows schematicallythe development of the boundary layer. This is greatly magnified forillustration purposes. The actual maximum thickness shown would be inthe range of 0.020". The diagram shows clearly the effects occurring inthe impingement zone and beyond. The boundary flow shown and existing infact has a 10W Reynolds number and is therefore laminar. Flow beyond theimpingement zone thus corresponds quite closely to the conditions offlat plate flow predicated by Pohlhausen, and as a result the boundarylayer thickness increases inversely as the square root of the distancetraveled. At some distance, generally in excess of one inch from theimpingement zone, the flow eventually becomes turbulent. It is clear,however, that any dust particle moving with the work toward thestagnation point would have great ditficulty in passing through thefalling thickness of the boundary layer and finally through theimpingement zone.

Velocity of impingement is also important, but only in relation to thetype and size of the dust particles and nature of the web surface. Forexample, large lightweight particles in the range of 100 microns in size(millionths of a meter) on a smooth surface free of electrostatic chargewould easily be picked up by using a velocity of 10,000 f.p.m. at thenozzles. On the other hand, fine particles in the range of 10 microns insize on the same surface would require as much as 25,000 f.p.m. nozzlevelocity. Whatever the particle size and type, however, the presentdevice is capable of doing the cleaning job many times more efficientlythan previous devices and is further capable of cleaning finer particlesfrom the surface of a web than was heretofore possible.

Various modifications are shown in the drawings. In FIG. 7, the primarynozzle 12 and plenum 8 are essentially as shown in FIG. 3. However, thesecondary jet has been completely eliminated and in lieu thereof thehousing 6 has its wall curved to be substantially parallel at 82 to theweb 60, which is here shown to be moving from right to left as in FIG.3. The movement of the web 60 beneath the closely parallel wall 82 tendsto carry ambient air into the exhaust chamber 36 thus serving, to adegree, as a substitute for the secondary nozzle in blocking escape ofdust-laden efiluent from the primary nozzle. While the device is not aseffective as that above described, it does enable the primary nozzle 12to achieve some of the advantages of the invention to a substantialdegree.

In the embodiment shown in FIG. 8, the primary nozzle 12 remainsunchanged and the wall 84 extends directly toward the web '60 and isprovided with a terminal brush 86 lightly brushing the web. To theextent that such a brush tends to loosen adherent particles of dust andalso to block the escape from the exhaust plenum 36 of air from nozzle12, the device is reasonably effective.

In FIG. 9 the construction is very closely similar to that of FIG. 8except that a rotary brush is used at 90, the wall 92 having its lowerterminal margin 94 curved around the cylindrical rotary brush toward theweb which repre-- sents the work. The rotary brush would tend to blockthe escape of air even if it were actuated with a peripheral speedcorresponding to the linear travel of the web but it preferably isoperated in the direction indicated by the arrow 96 and at a speed suchthat the peripheral speed of the brush materially exceeds the rate oflinear web travel.

The embodiment shown in FIG. uses the same nozzle arrangement shown inFIG. 3 but gives normally unnecessary electrostatic assistance fordislodging dust. As the web approaches the cleaner, it passes a wipingplate 98 which imparts an electrostatic charge to particles on thesurface of the moving web. In the area between the nozzles 12 and 14,and beneath the web, there is a field plate 100 which has a similarcharge. Spaced slightly over the web within the exhaust plenum 36 is atransversely extending electrode 102 that is oppositely charged. Thewiring is schematically illustrated.

It is desired to indicate by the various modifications disclosed thatthe method invention is not limited to the use of any particularphysical structure. At the same time, it is desired to emphasize thatthe various alternatives shown are not by any means intended to exhaustthe possibilities for modification. The essential feature of the methodis a primary jet having the characteristics above described andpreferably used in the specified relation to the moving web and with asecondary jet and with the step of confining and removing the dust-ladenair resulting from dislodgement of dust from the boundary layer on thework.

I claim:

1. A method of cleaning dust and other particle material from alongitudinally moving web, said method comprising the steps of directinga transversely elongated gas jet of at least 10,000 f.p.m. against theweb in a direction substantially normal to one face thereof so as tointerrupt the gaseous boundary layer adjacent to the web, dischargingthat component of the jet which moves downstream with the web afterimpingement thereon, directing onto the web and from a position upstreamtherefrom a second transverse gas jet in a direction at an acute angleto the movement of said web and toward said first-mentioned jet in thedirection of web movement, and collecting from the vicinity of the webbetween the gas jets the gas from the second-mentioned jet and thatcomponent of the gas of the first-mentioned jet which travels counter tothe direction of web movement, the first mentioned gas jet originatingat a distance from the web which is no more than 1.0 inch therefrom andthe ratio of its cross sectional area at its origin to the distance ofits point of origin from the web being within the range of 0.02 to 1 and0.15 to 1, and the angle of the second gas jet with respect to the webbeing within a range of 5 to 25 degrees.

2. A method according to claim 1 in which the first jet originates atabout A" from the web.

3. A method according to claim 1 in which the first and second jets aresufiiciently close in the direction of web travel so that each retainssubstantial energy when the flows of the respective jets meet, themomentum of flow being substantially balanced.

References Cited UNITED STATES PATENTS 1,196,437 8/1916 Doyle 15-306.11,658,485 2/1928 Hayward 15307 2,022,593 11/1935 Fuykers 15-345 XR2,139,628 12/1938 Terry l5316 XR 2,358,334 9/1944 Knowlton 15--309 XR2,482,781 9/ 1949 Knowlton et al. 15303 XR 2,515,223 7/1950 Hollick15306.1 XR 2,648,089 8/1953 Mayer 15306.1 2,920,987 1/1960 Landry et a1.151.5 XR 2,956,301 10/1960 Bruno 15306.1 3,045,273 7/1962 Bruno 15306.1

FOREIGN PATENTS 732,972 7/ 1955 Great Britain.

MORRIS O. WOLK, Primary Examiner.

I. ZATARGA, Assistant Examiner.

US. Cl. X.R. 134-1, 15, 21

